General theory, history and development

Abstract

Echocardiography (M-mode) was first described by Dr. Inge Edler (Fig. 2.1) working at the university of Lund, Sweden, in 1954. The term echocardiography refers to a group of tests that use ultrasound to examine the heart and record information in the form of echoes, i.e., reflected sonic waves.1 The upper limit for audible sound is 20 KHz. The sonic frequency used for echocardiography ranges from 1 to 10 MHz (these are known as ultrasound waves). The characteristics of a sound wave can be expressed as a sine wave. The distance between two similar areas along the wave path (e.g., the distance that wave travels during a single cycle) is termed a wavelength (λ). The number of the wavelengths per unit time is the frequency (f) of the sound wave, which is measured in cycles per seconds (cps, also called Hertz [Hz]). One of the fundamental principles in sound physics is:

$$c = f\lambda $$

(1)

where c is the speed of sound in a medium, f is the frequency of the sound wave, and λ is the wavelength of the sound in the medium. The frequency of the sound depends only on the source that is producing it, but the speed of sound (and hence of wavelength) varies from one medium of propagation to another.1